On a seat of an automobile and the like, a seat belt device is mounted in order to protect an occupant at the time of emergency, such as collision. For easy insertion and removal of a seat belt device of this type, a buckle device is equipped in a seat of an automobile and the like. A buckle device is configured to spring bias a latch member that latches a tongue plate in a latching direction and also to use a locking member to hold the latch member in a state in which the tongue plate and the buckle main body are latched. In such a buckle device, it is required to indicate by a lamp that the tongue plate and the buckle main body are in a latched state, to control actuation of a belt winding power reduction mechanism and the like, or to enable transmission of information of insertion and removal of a seat belt to an air bag ECU (electronic control unit). Therefore, a buckle switch that detects a latched state is equipped inside the buckle. As a prior art related to a buckle switch of this type, there are, for example, Patent Literature 1 shown below and the like. Among buckle switches, there is a switch of a type that detects only a state of contact point conduction/opening at the time of insertion and removal (hereinafter, referred to as an ON/OFF detecting buckle switch).